The present invention relates to an ink-jet printer which prints an image onto a print medium held on a rotary drum with ink ejected from a print head, and particularly, to an ink-jet printer whose print head is constructed by a plurality of ink-jet nozzles disposed in the axial direction of the rotary drum.
Conventionally, serial-type ink-jet printers are widely spreading. In the serial-type ink-jet printer, a print head and an ink cassette of a relatively small capacity are integrally mounted on a carriage, and the carriage is movably attached to a guide bar extending across a paper sheet. The paper sheet is fed in a direction perpendicular to the guide bar at a constant pitch, and the carriage is moved along the guide bar each time the paper sheet is fed for one pitch. The print head ejects ink during the movement of the carriage. In the case where the printer is used for color printing, the print head includes a plurality of ink-jet nozzles which are respectively supplied with inks of different colors from ink tanks. In the structure as described above, for example, a color image of A4 size is printed out in ten minutes. Thus, the serial-type ink-jet printer operates at a slow print speed of 0.1 sheet per minute.
In recent years, a drum rotation type ink-jet printer has been developed to perform color printing at a higher speed. In this ink-jet printer, a paper sheet is held on a rotary drum rotating in only one direction, and a print head includes a plurality of nozzle units which are arranged along the peripheral surface of the rotary drum and eject inks of different colors other onto a paper sheet rotating together with the rotary drum. Each nozzle unit includes a plurality of ink-jet nozzles disposed across the paper sheet in the axial direction of the rotary drum. The pitch of the ink-jet nozzles is set to a value equal to a desired resolution or a value two to four times greater than the resolution. The print head is positioned such that the end surfaces of the ink-jet nozzles are close to the paper sheet on the rotary drum. The print head is set to a predetermined position in the case where the pitch of the ink-jet nozzles is equal to the desired resolution. The print head is set to be movable in the axial direction of the drum from the predetermined position in the case where the pitch of the ink-jet nozzles exceeds the desired resolution. When the print head is movable in the axial direction of the rotary drum, the print head is moved at a rate corresponding to the desired resolution, for each revolution of the rotary drum, and is returned to the predetermined position after the print head is moved for a distance equal to the pitch of the ink-jet nozzles. The rotation speed of the rotary drum is set to 120 rpm. In this structure, for example, a color image of A4 size can be printed out in about two or three seconds. Also, since the print head is not moved by a distance exceeding the nozzle pitch in the axial direction of the rotary drum, the number of prints to be obtained for each ink charge can be increased by setting large-capacity ink cassettes apart from the print head and supplying inks of different colors to the respective nozzle units of the print head.
In this ink-jet printer, the end surface of the print head corresponding to the end surfaces of all the ink-jet nozzles are close to a paper sheet with a gap of about 1 mm interposed therebetween. Therefore, during printing in which a paper sheet is rotated at a high speed by a rotary drum and moved relatively with respect to the ink-jet nozzles, paper particles scattered from the paper sheet easily adhere to the end surface of the print head. The paper particles are gradually accumulated and soak up ink on the end surfaces of the nozzles. If such paper particles drop on a paper sheet along with ink, the print quality is degraded. The degradation of the print quality is a more serious problem for a drum rotation type ink-jet printer in which the print head is used for a long period than for a serial type ink-jet printer in which the print head is replaced upon shortage of ink in an ink cassette of a small capacity.
However, since the gap between the end surface of the print head and a paper sheet is slight, it is difficult to remove safely and securely paper particles adhered to the end surface. Therefore, for example, a cleaning process may be performed at the non-printing time to remove the paper particles by moving the print head in the axial direction of the rotary drum from a printing position facing the peripheral surface of the rotary drum to a cleaning position not facing the peripheral surface of the rotary drum, and mechanically wiping the end surface of the print head with an elastic material such as rubber upon movement of the print head. In this case, the size of the drum rotation type ink-jet printer will be increased in accordance with the distance of moving the print head. Further, an increase of the print speed is hindered by time losses caused by moving the print head between the cleaning position and the printing position. In this respect, since the end surface of the print head is coated with a water repellent film so that ink is ejected from ink-jet nozzles through predetermined courses onto a paper sheet, the pressure to the elastic material and the moving speed of the print head must be appropriately restricted.